<font color="yellow">and you can see the flames climbing up half the first stage. <br />My question: Why? and Wasn't it dangerous.... ?</font><br /><br /><br />Late reply here, but these are interesting photographs.<br /><br /><font color="yellow">WHY?</font><br />What you're seeing is a combination of interesting aerodynamics and combustion phenomena that makes it unique on the Saturn vehicle. <br /><br />First the aerodynamic effect, the relatively high engine exit pressure from the 5 F-1 engines created a large enough plume that forced "oblique shocks" from the air flowing down the first stage. This creates a "flow separated" region where the higher pressure from the plume creates a "back-flow" into this separation zone and recirculates.<br /><br />Second, the above flow separation coupled with the fuel-rich exhaust from the F-1, which is fuel rich (left with unburned fuel) that "back flow" into this zone rich with oxygen and with a pressure high enough to encourage combustion, it then burns and flows out along the "oblique shock" line.<br /><br />This is interesting and unique because a single engine vehicle usually does not produce a strong enough "back-pressure" to cause this phenomena. But multiple engines, esp. a combination of 5 engines, creates a strong "jet pump" effects in between the engines. When it "entrains" air, the air accelerates thus further lower its static pressure which further encourages the engine plume to "back flow" foward.<br /><br /><font color="yellow">Wasn't it dangerous.... ?</font><br />It could be if you have a easily flammable materials in that zone. But fortunately for the Saturn, as well as most launch vehicles, that there are usually insulations in that area and they are not easily burn. So it is mostly safe.<br /> <div class="Discussion_UserSignature"> </div>